This invention is directed to an absorbent composition containing transitional crosslinking points. There are two types of crosslinking points, conventional permanent crosslinking points and transitional crosslinking points. Permanent crosslinking points are those that exist in a polymer without undergoing a significant increase or decrease in their total numbers before, during, or after saline saturation. Transitional crosslinking points can either be pre-existing in a polymer and undergo a significant decrease in their total numbers after saline saturation, or can be established in a use situation and undergo a significant increase in their total numbers during or after saline saturation.
Superabsorbent materials used in current disposable diapers or other personal care products are generally a crosslinked highly water insoluble but swellable polyelectrolyte. For example, a high molecular weight sodium polyacrylate salt (Na—PA), crosslinked by either covalent bonds, such as —C—C—, —C—O—, —C—N—, or ionic bonds, such as Al3+, Zr4+, Fe3+, Cr3+, Ti3+, or Ce4+, can absorb more than 40 grams of 0.9% NaCl saline per gram of the polymer when no external pressure is applied on it, or more than 20 grams of the saline per gram of the polymer when a 0.3 pound per square inch pressure is applied on it. The absorbency without pressure on the polymer is the free swell capacity, and that with pressure on it is the absorbency under load (AUL).
In general, a polyelectrolyte, when crosslinked slightly, has a high free swell capacity but a low AUL value due to a low gel stiffness. On the other hand, a polyelectrolyte, when crosslinked heavily, has a lower free swell capacity but a higher AUL value. In order to have a maximum AUL value, current superabsorbent material has to sacrifice its free swell capacity. This is only true when the permanent crosslinking points are formed in the superabsorbent material. Though a high free swell but soft gel is not capable of absorbing liquid under pressure, it is capable of retaining huge amounts of liquid under pressure if the gel is allowed to swell first and then a load is applied.
Current commercially available superabsorbent materials are typically crosslinked by permanent crosslinking points by covalent bonds. It is known in the art to use metal ions (Al3+ or Zr4+) as either bulk or surface crosslinking agent. Within the same art, the addition of the metals ions into absorbent polymers is followed by a drying process. The drying process causes the resulting ionic bonds to be permanent crosslinks rather than transitional crosslinks.
Because conventional superabsorbent material is incapable of simultaneously possessing high free swell capacity and high AUL, it can be difficult to control the fluid absorption rate of the material. Furthermore, the process of making the superabsorbent material can be complicated due to the necessity of carrying out a surface crosslinking step or other modification processes in order to adjust the fluid absorption rate, fluid distribution, and fluid intake.
There is a need or desire for an absorbent material that can simultaneously possess high absorbent capacity and high absorbency under load.